Thin film transistors (TFTs) have been widely used as pixel switching elements in display devices such as liquid crystal displays and EL displays. In recent years, a pixel array driver circuit using TFTs has been increasingly formed on the same substrate. Conventionally, such TFTs have been formed on a glass substrate using amorphous or polycrystalline silicon.
A CVD apparatus used for forming such a TFT using silicon is very expensive and an increase in the area of the display device using TFTs leads to a substantial increase in manufacturing costs. In addition, since a process for forming a film of amorphous or polycrystalline silicon is performed under extremely high temperatures, only materials of high heat resistance such as a glass substrate can be used as a substrate.
Against this backdrop, TFTs using CNTs as active layers have been proposed. As a method of forming the TFTs using CNTs, a wet process such as an application method in which a CNT-dispersed solution in which CNTs are dispersed is applied onto the substrate, followed by drying. Such wet process requires no expensive device such as a CVD apparatus and can achieve an increase in the area at low costs. Furthermore, since the process temperature is low, advantageously, there are only reduced limitations in selecting a material for the substrate. TFTs using such a CNT-dispersed film is disclosed in the following prior art documents (1) to (6):    (1) E. S. Snow, J. P. Novak, P. M. Cambell, D. Park, “Random networks of carbon nanotubes as an electronic material, Applied Physics Letters, vol. 82, No. 13, p.p. 2145-2147 (2003)    (2) E, Artukovic, M. Kaempgen, D. S. Hecht, S. Roth, G. Gruener, “Transparent and Flexible Carbon Nanotube Transistors”, Nano Letters, vol. 5, No. 4, p. 757-760    (3) S.-H. Hur, O. O. Park, J. A. Rogers, “Extreme bendability of single-walled carbon nanotube networks transferred from high-temperature growth substrates to plastic and their use in thin-film transistors”, Applied Physics Letters, vol. 86, p. 243502 (2005)    (4) T. Takenobu, T. Takahashi, T. Kanbara, K. Tsukagoshi, Y, Aoyagi, Y. Iwasa, “High-performance transparent flexible transistors using carbon nanotube films”, Applied Physics Letters, vol. 88, p. 33511 (2006)    (5) Japanese Unexamined Patent Publication No. 2005-347378    (6) Japanese Unexamined Patent Publication No. 2006-73774
For TFTs in which the active layer is in contact with the gate dielectric film, the CNT gets into contact with the gate dielectric film when CNTs are used as the active layer. Known materials for the gate dielectric film include, as disclosed in the prior art reference (2), for example, high polymer materials selected from polyester, polyvinyl, polycarbonate, polypropylene, polyethylene, polyacetate, polyimide or dielectrics thereof.
When an attempt is made to form the active layer including CNTs to be in contact with a gate dielectric film using a conventional gate dielectric film, however, there are cases in which an interaction between the gate dielectric film and the CNTs are not sufficiently obtained. When a solution containing CNTs is applied on the gate dielectric film and dried to form TFTs, for example, there are cases where an interaction between the CNTs in the solution and the surface of the gate dielectric film is insufficient and only an insufficient amount of CNTs are fixed as the active layer, so that sufficient density and uniformity are not obtained. Such uniformity of the amount of fixed CNTs in the active layer leads to variations in switching characteristics of the element and the yield of arrayed elements is largely lowered. To solve this problem, a measure may be taken in which the channel region is surface-treated with aromatic thiol such as mercaptopyridine to fix CNTs at high density. However, according to this measure, the manufacturing process becomes complicated, because, an additional finishing process is required and a patterning process must be added to prevent adhesion to places other than the channel region, thereby contributing to an increase in costs.
For a case where the active layer containing CNTs is previously provided and the gate dielectric film is formed on the active layer, on the other hand, when an interaction between the CNT and the gate dielectric film lacks, it contributes to instability of the interface between the active layer and the gate dielectric film, which becomes one of factors to cause change with time of element characteristics.